**Core Concept**
The radial nerve is a major nerve in the arm that supplies various muscles, including those involved in wrist and finger extension. It runs through the spiral groove of the humerus, where it can be injured, affecting its supply to muscles below this point. The **radial nerve** supply to muscles is crucial for understanding the clinical presentation of radial nerve palsy.

**Why the Correct Answer is Right**
The correct answer choice should be a muscle not supplied by the radial nerve below the spiral groove. Typically, the radial nerve supplies the **extensor muscles of the wrist and fingers**, such as the extensor carpi radialis brevis, extensor carpi ulnaris, and the extensor digitorum, among others. The muscle not supplied by the radial nerve below the spiral groove would be one that is either supplied by a different nerve or by a branch of the radial nerve that originates above the spiral groove.

**Why Each Wrong Option is Incorrect**
**Option A:** This would be incorrect if it's a muscle typically supplied by the radial nerve below the spiral groove.
**Option B:** Similarly, if this option is a known target of the radial nerve's motor branches below the spiral groove, it cannot be the correct answer.
**Option C:** If this choice represents a muscle known to be innervated by the radial nerve after it exits the spiral groove, then it's not the correct choice.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the radial nerve's supply to the extensor muscles of the wrist and fingers is critical for the clinical presentation of radial nerve palsy, which includes wrist drop. Understanding which muscles are supplied by the radial nerve and at what level can help in diagnosing and managing such conditions.

**Correct Answer:** D. Triceps